1. Field of Invention
This invention relates to a new switch control method for the well-known bridge circuit, where two semiconductor switches and two flywheel diodes are connected in series in a bridge between a positive and a negative power supply rail. The invention also relates to a bridge circuit for performing said method.
2. Prior Art
A bridge circuit of the above-mentioned type usually comprises one or more sets of components and one such set of components is normally called one "leg" of the bridge circuit and sometimes a "half-bridge". Very often two legs are used in a "full-bridge", with the load connected between the two legs which are driven with opposite polarities. In three-phase systems, e.g. three-phase motor control circuits, three legs are used as is well-known. The present invention discloses one leg of a bridge circuit, whereby this leg may be used as a building block in all types of systems with any number of legs.
The new method and the new switch circuit according to this invention are especially adapted for using the POWER MOSFET transistor, where a flywheel diode is an integral part of the MOSFET transistor. However, the circuit may as well be used with any other type of semiconductor, such as bipolar transistors or thyristors, with external flywheel diodes.
The POWER MOSFET transistor, in the following simply named transistor, has an integral "reverse diode", which can be used as a flywheel diode in the above-mentioned bridge connection. This is very favourable, since it minimizes the number of circuit components.
However, the use of said reverse diode as a fly-wheel diode is not free from problems. Such an application which may cause trouble is the pulse width modulated inverter for AC motor drives having inductive load. The load current changes slowly, and may have the same polarity and be approximately constant during multiple bridge output pulses.
Suppose in such a case, that the upper transistor in a bridge circuit has been turned ON and has supplied a positive output current to the load. When the transistor is turned OFF, the inductive load current must find a new way through the lower flywheel diode. But, in this type of application, the flywheel current does not go to zero. The upper transistor must be turned ON while the lower flywheel diode is still conducting. Because of the internal transistor structure, details of which are not discussed here, the lower transistor may be unintentionally turned ON when the upper transistor turns ON which causes a bridge short circuit with catrastrophic outcome. These are well-known facts and are described in the literature, see e.g. RCA Power MOSFET's Databook, pages 493-499 (1986), INTERNATIONAL IP. RECTIFIER Power MOSFET HEXFET.RTM. Databook, pages A-74-A-76 (1985), MOTOROLA SEMICONDUCTORS TMOS.RTM. Power MOSFET Transistor Data, pages A-30-A-31 (1985), and SIEMENS SIPMOS.RTM. Datenbuch 1984/85, pages 19-20.
Even with separate external flywheel diodes, the turn ON of the transistor is critical. In the turn ON moment, the transistor has to supply current to the output, and simultaneously supply reverse recovery current to the opposite diode. If the turn ON is fast, as it should be, it will be necessary for the transistor to supply more than twice the load current during the turn ON moment.
Some transistor manufacturers have designed special transistor versions for such applications, e.g. SIEMENS with the FREDFET (Fast-Recovery-Epitaxial-Diode-Field-Effect-Transistor). The internal reverse diode has been modified to a fast recovery diode, while the reverse diode normally used has a relatively slow recovery. This measure may partially overcome the problem, although it does not seem to remove the real source of problem.